1. Field of the Invention
The present invention relates to chemical sensors, and particularly to a method for chemical vapor identification using swelling-based sensors (chemiresistors).
2. Description of the Related Art
Chemiresistors are a class of chemical sensors wherein the electrical resistance of a sensing element changes in response to the presence or absence of a chemical species. Chemiresistors are attractive for use as chemical sensors, since it is relatively easy to measure and record small changes in electrical resistance. Swelling-based chemiresistor sensors use composites, typically, thin films of an electrically insulating matrix, filled with conductive particles. Sensitivity to analytes results from the swelling of the matrix (usually a polymer thin film) in the presence of analyte vapors, a consequential increase in the separation of conductive particles and a resulting increase of electrical resistance, R, or, decrease of conductance, G, which is monitored readily. The traditional implementation of swelling-based vapor sensors are insulating polymers filled with carbon black (CB) particles. Selectivity is due to the choice of insulating matrix, as matrices will swell most in vapors of similar polarity and polarizability, which is quantified by the Hildebrand parameter (δ).
For example, to distinguish between a range of solvent vapors, as in U.S. Pat. No. 5,571,401, issued Nov. 5, 1996 to Lewis et al., an array of chemiresistors was prepared using different polymer matrices, most of which were rather polar (high δ). Solvents were identified by careful analysis of the response pattern of the array. The use of an array of generic sensors with subsequent analysis of response pattern as a broad-range vapor identification system is akin to mammalian olfaction systems (noses), albeit these do not use swelling-based chemiresistors.
More recently, the concept of swelling-based sensors has been extended to films of core/shell nanoparticles (CSNPs), e.g., using organothiol ligand shells coupled to gold nanoparticle cores via Au/thiol coupling. Although such films work as swelling-based gas sensors with good response to alkane and other vapors, it cannot discriminate well between different alkanes. Similarly, the commercially established ‘pellistor’ catalytic sensors are sensitive to all flammable gases, thus warning of explosion hazards, but it is without the ability to discriminate between different alkanes. Even large arrays of swelling-based sensors cannot discriminate well between chemically similar vapors, more precisely, vapors with similar Hildebrand parameter. However, as swelling is a generic interaction, discrimination between different vapors with swelling-based sensors poses a challenge.
Thus, a method for chemical vapor identification using swelling-based sensors solving the aforementioned problems is desired.